1. Field of the Invention
The present invention relates to negative temperature coefficient thermistors (hereinafter referred to as NTC thermistors), and particularly relates to a multilayer NTC thermistor including internal electrodes and a method for manufacturing such a thermistor.
2. Description of the Related Art
Demands have been made for NTC thermistors, intended for temperature sensors and temperature compensators, having low resistance. To achieve that, the following technique, for example, is disclosed in Japanese Unexamined Patent Application Publication No. 4-328801: Cu is added to an NTC thermistor element comprising a sintered body of a spinel metal oxide containing Mn, Co, Ni, and so on, thereby reducing the resistivity.
The following technique is disclosed in Japanese Patent No. 3218906: an external electrode material containing Cu is applied to end faces of an NTC thermistor element and a Cu component contained in electrodes is localized at the interface between each electrode and the element to reduce the resistivity.
These conventional techniques are intended for lead-type NTC thermistors. When the techniques are used for chip-type NTC thermistors, problems arise.
In Japanese Unexamined Patent Application Publication No. 4-328801, as shown in FIG. 1, a first NTC thermistor 1 includes a first NTC thermistor element 2 and first external electrodes 3, disposed on both ends of the first NTC thermistor element 2. When a ceramic composition containing Cu is used for forming the first NTC thermistor element 2, the first NTC thermistor element 2 uniformly contains Cu and thus the entire first NTC thermistor element 2 has low resistivity. Therefore, there is a problem in that a metal coating is formed on the first NTC thermistor element 2 when metal coatings are each formed on corresponding first external electrodes 3 by an electrolytic plating process.
In Japanese Patent No. 3218906, as shown in FIG. 2, a second NTC thermistor 11 includes a second NTC thermistor element 12, having a chip shape, and second external electrodes 13. When Cu is added to an electrode-forming material such that Cu migrates from electrodes to the second NTC thermistor element 12 by diffusion, formed are regions A of the second NTC thermistor element 12 having a resistivity smaller than that of other regions, regions A being adjacent to the second external electrodes 13. Therefore, there is a problem in that a metal coating is formed on the second NTC thermistor element 12 when the electrode-forming material containing Cu is applied to both ends of the NTC thermistor element 12, the second external electrodes 13 are formed by firing the resulting material, and metal coatings are then formed on the corresponding second external electrodes 13 by an electrolytic plating process. This is because regions a of the second NTC thermistor element 12 function as cores from which coatings grow to form the metal coating.
In order to solve the above problems of the conventional techniques, the following chip-type thermistor has been proposed, as shown in FIG. 3: a third NTC thermistor 21 including a third NTC thermistor element 21, third internal electrodes 24 disposed in the third NTC thermistor element 22, and third external electrodes 23 disposed at both ends of the third NTC thermistor element 22 and electrically connected to the third internal electrodes 24. However, even if a material for forming the third external electrodes 23 contains Cu, the quantity of diffused Cu is insufficient to control the resistance although Cu is diffused in the third NTC thermistor element 22 from the third internal electrodes 24. Thus, the resistance of the third NTC thermistor 21 cannot be sufficiently decreased.